Stimulating and lesioning electrodes are used in a variety of surgical procedures, in particular, DBS electrodes are used in a variety of neurosurgical procedures.
A surgeon wishing to stimulate or lesion a particular area of nervous tissue, can target the end of an electrode to the target site so that a desired electrical current can be delivered. Numerous methods are known for targeting the electrode to the desired site including stereotactic methods.
Generally, deep brain stimulating electrodes are manufactured by forming a coil of one or more insulated wires having non-insulated ends on a support, welding electrode conducting areas on to the non-insulated ends of the wires and placing a sheath of non-conducting material over the non-conducting parts of the electrode. It is clear that such a method for producing an electrode is laborious and therefore expensive.
Furthermore, as numerous parts are used in the construction of the electrode, it is possible that the overall diameter of the electrode will vary along its length. In particular, the electrode areas which are welded on to the electrode, especially to spot weld points, can be proud of the rest of the surface of the electrode leading to difficulties in inserting the electrode. A further problem with electrodes constructed in this manner is that the electrode has to be of a sufficient size for it to enable electrode conducting areas to be welded onto the non-insulated ends of the wires.
There is therefore a need in the art for an electrode which can be constructed more efficiently and with greater accuracy.
It is becoming increasingly common for patients with disorders of brain function, including disorders of movement, intractable pain, epilepsy and some psychiatric disorders to be treated with deep brain stimulation. DBS electrodes are chronically implanted into the fine targets in the brain where electrical stimulation will disrupt abnormal neural firing in these patients to alleviate their symptoms. Brain targets for treating functional disorders are usually deeply situated and of small volume. For example, the optimum target for treating Parkinson's disease is situated in the sub-thalamic nucleus (STN) and is a sphere of 3 to 4 mm in diameter or an ovoid of 3 to 4 mm in diameter and 4 to 5 mm in length. Other targets such as the globus pallidus (used for treating hyper- or hypo-kinetic disorders) or targets in the thalamus (used for treating tremor) are usually no more than 1 to 2 mm larger.
Current DBS electrodes, for example those supplied by Medtronic Inc, Minneapolis, Minn., are of dimensions to accommodate such volumes. For example, such electrodes have a diameter of about 1.27 mm and have 4 ring electrodes of the same diameter positioned at their distal end. Each ring electrode has a length of 1.5 mm with a 1.5 or 0.5 mm separation. In use, the DBS electrode is connected to a battery driven pulse generator via a cable and the equipment implanted subcutaneously, generally with the pulse generator positioned below the clavicle. The frequency, amplitude and pulse width of the stimulating current delivered to the electrode contacts can be programmed using external induction.
A problem with the use of such electrodes is the difficulty in accurately placing the electrode within the desired target. The accuracy of placement is key to the effectiveness of the treatment. For a small target such as the STN, misplacement of the electrode by no more than 1 mm will not only result in sub-optimal symptomatic control but may induce unwanted side effects such as weakness, altered sensation, worsened speech or double vision (see FIG. 4).
The established method to place an electrode into a functional brain target is first to localise the area of abnormal brain function. This is achieved by fixing a stereotactic reference frame to the patient's head, which can be seen on diagnostic images, and from which measurements can be made. The stereotactic frame then acts as a platform from which the electrode is guided to the target using a stereoguide that is set to the measured co-ordinates.
However, functional neurosurgical targets are often difficult or impossible to visualise on diagnostic images and so their actual position may need to be inferred with reference to visible land marks in the brain and using a standard atlas of the brain to assist the process. Due to anatomical variation between an individual and the atlas and even between different sides of the same brain in an individual such differences can lead to error in target localisation. Errors in target localisation may also result from patient movement during image acquisition or geometric distortion of images which can be intrinsic to the imaging methods. Such errors may be further compounded at surgery by per-operative brain shift. This may result from the change in head position from that during image acquisition to the position on the operating table, from leakage of cerebrospinal fluid when a burr hole is made with subsequent sinking of the brain and/or from the passage of the electrode through the brain substance. Surgeons attempt to correct these errors by performing per-operative electrophysiological studies on the patients undergoing functional neurosurgery who are kept awake during the procedures. These studies include microelectrode recording of the neural firing in the planned target area and/or stimulation of the target area using a test electrode. A series of passes are made through the target area with microelectrodes and sample recordings taken. The target is defined by its characteristic patterns of firing. Because of the jelly-like consistency of the brain and the depth of the functional targets within it, there needs to be a space of about 2 mm between different microelectrode passes to prevent the electrode passing down a previously made track. Thus, for a small target such as the STN, it is possible for the recordings from two microelectrode passes, 2 mm apart, to both register location within the target structure but to find neither of them to be optimally located centrally within the target. Likewise, if a test stimulation electrode is passed just off the optimal target position, i.e. .+−.1 millimeter, then a second pass to correct this error will almost inevitably result in the electrode passing down the same track.
If an electrode is placed exactly in the centre of a target having a 3 mm diameter, then the distance from the electrode surface to the edge of the target is usually under 1 mm. If the current spreads beyond this, then side effects can be incurred. For these reasons, given the small chance that an electrode will be placed in the centre of a target and that a placement error of .+−.1 mm can result in sub-optimal treatment with side effects, which cannot readily be corrected with repositioning, there is a need for an electrode which overcomes at least some of these problems.
U.S. Pat. No. 5,843,148 discloses a high resolution brain stimulation lead, wherein the electrode comprises ring segments diagonally arranged along the circumference of the lead. Accordingly, in theory by passing a stimulation current between electrode contact areas (i.e. ring segment), off axis stimulation can be achieved. Off axis stimulation refers to the generation of an electric field that is displaced to one side of the electrode. Furthermore, by rotating the lead, different volumes of tissue around the lead circumference may be stimulated. The major problem with this device is that the diagonal geometry of the ring segment results in a complex electric field which will spiral around the portion of the diameter of the electrode and is necessarily elongated along the axis of said electrode. The proposed configuration would therefore not form an off axis electric field that is suitable for treating a desired target. Furthermore, this device does not enable one skilled in the art to adjust the volume of tissue being stimulated in both the axial plane and the horizontal plane independently. Instead, on rotating the electrode, the volume of tissue stimulated varies in both the horizontal and axial planes, making interpretation of patient's responses extremely difficult. Furthermore, the complex geometry of the proposed electrode would be difficult to construct and vulnerable to mechanical failure.
There is therefore a need for an electrode which overcomes at least some of the problems associated with the prior art electrodes.